Q：

Given two binary trees, write a function to check if they are equal or not.
Two binary trees are considered equal if they are structurally identical and the nodes have the same value.

A:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isSameTree(TreeNode p, TreeNode q) {
       if (p == null && q == null) return true;
       if (p == null || q == null) return false;
       if (p.val != q.val) return false;
       return isSameTree(p.left, q.left) && isSameTree(p.right, q.right); //迭代！ 
    }
}